Conventional complementary metal-oxide-semiconductor (CMOS) devices offer higher on-current for n-type field effect transistors (NFETs) than for p-type field effect transistors (PFETs) having similar physical dimensions. This is, in general, due to higher electron mobility than hole mobility in most semiconductor materials. In the case of a silicon substrate having a (100) surface, the ratio of electron mobility to hole mobility is about 2. Correspondingly, semiconductor circuits have been designed to factor in the differences in the on-current per unit width of NFETs and PFETs.
Static random access memory (SRAM) is a memory device employing six transistors. SRAM cell design typically begins by picking the smallest PFET supported by a given technology for two pull-up PFETs, followed by scaling of NFET pass gate transistors and pull-down NFET transistors for optimal beta ratio, cell stability, and access time.
Referring to FIG. 1A-1C, an exemplary prior art SRAM structure comprises a first pull-up PFET 16, a second pull-up PFET 16′, two pull down NFETs (14, 14′), and two pass gate NFETs (12, 12′). FIG. 1A is a top-down view of the exemplary prior art structure up to the CA level not showing a middle-of-line (MOL) dielectric 70. FIG. 1B is a vertical cross-sectional view of the exemplary prior structure along the plane B-B′ showing the MOL dielectric 70. FIG. 1C is a vertical cross-sectional view of the exemplary prior structure along the plane C-C′ showing the MOL dielectric 70. Each of the transistors (12, 12′, 14, 14′, 16, 16′) comprise a portion of the semiconductor substrate 10, a gate dielectric 30, a gate conductor 32, a gate spacer 34, active area (AA) silicides 60, and gate top silicides 64. Shallow trench isolation 20 physically separates the transistors (12, 12′, 14, 14′, 16, 16′) and provides electrical isolation among the transistors (12, 12′, 14, 14′, 16, 16′). CA contact vias 76 and CA bars 78 are employed to provide electrical wiring among the transistors (12, 12′, 14, 14′, 16, 16′). One of the CA bars 78, which contacts one of the AA silicides 60 of the first pull-up PFET 16 as well as the gate top silicides 64 of the second pull-up PFET 16′ as shown in FIG. 1B, provides electrical connection between the drain of the first pull-up PFET 16 and the gate of the second pull-up PFET 16′. Likewise, another CA bar 78 provides electrical connection between the drain of the second pull-up PFET 16′ and the gate of the first pull-up PFET 16.
Referring to FIGS. 2A-2C, the exemplary prior art SRAM structure is shown up to the M1 level. FIG. 2A is a top-down view of the exemplary prior art structure up to the M1 level not showing the middle-of-line (MOL) dielectric 70 and an M1 dielectric 80. FIG. 2B is a vertical cross-sectional view of the exemplary prior structure along the plane B-B′ showing the MOL dielectric 70 and the M1 dielectric 80. FIG. 2C is a vertical cross-sectional view of the exemplary prior structure along the plane C-C′ showing the MOL dielectric 70 and the M1 dielectric 80. M1 wires 88 embedded within the M1 dielectric 80 contact the underlying CA contact vias 76 and the CA bars 78. In the exemplary prior art SRAM structure, FIGS. 2A and 2C illustrate that the drain of each of the two pull-up transistors (16, 16′) is electrically connected to a node at which a source/drain of one of the pass gate transistors (12, 12′) adjoins the drain of one of the pull-down NFETs (14, 14′) by a combination of a CA bar 78, an M1 wire 88, and a CA contact via 76. Two such combinations are present in each SRAM cell structure which comprises six transistors (12, 12′, 14, 14′, 16, 16′).
Referring to FIG. 3, a circuit schematic 18 for the exemplary prior art SRAM structure shows a first pair of a first pass gate n-type field effect transistor (NFET) 2 and a first pull-down n-type field effect transistor (NFET) 4 wherein a first source/drain of the first pass gate NFET 2 and a first drain of the first pull down NFET 4 are adjoined to form an electrical connection. In the physical structure, this electrical connection is achieved by a first common active area that contains both the first source/drain of the first pass gate NFET 2 and the first drain of the first pull-down NFET 4. Similarly, a second source/drain of the second pass gate NFET 2′ and a second drain of a second pull-down NFET 4′ are adjoined to form another electrical connection. In the physical structure, this electrical connection is achieved by a second common active area that contains both the second source/drain of the second pass gate NFET 2′ and the second drain of the second pull-down NFET 4′. The circuit schematic 18 further comprises a first pull-up p-type field effect transistor (PFET) 6 containing a third drain, which is physically a third active area, and a second pull-up PFET 6′ containing a fourth drain, which is physically a fourth active area. Each of the source/drain nodes of the pass gate transistors (2, 2′) may function as a source or a drain depending on the operation of the SRAM circuit.
The third active area is electrically connected to the first active area via a collection of a first contact via, a first M1 wire, and a first CA bar. This connection is represented in the circuit schematic 18 by a first internal node 11. Similarly, the fourth active area is electrically connected to the second active area via a collection of a second contact via, a second M1 wire, and a second CA bar. This connection is represented in the circuit schematic 18 by a second internal node 11′. The gates of the second pull-up PFET 6′ and the second pull-down NFET 4′ are adjoined to the third drain of the first pull-up PFET 6 via the first CA bar. This connection is represented in the circuit schematic 18 by a third internal node 13A and a fourth internal node 13B. The gates of the first pull-up PFET 6 and the first pull-down NFET 4 are adjoined to the fourth drain of the second pull-up PFET 6′ via the first CA bar. This connection is represented in the circuit schematic 18 by a fifth internal node 13A′ and a sixth internal node 13B′. The internal nodes (11 11′, 13A, 13B, 13A′ 13B′) are connected by CA contact vias 76 and CA bars 78 as well as M1 wires 88. Bit line wiring (15, 15′) and word line wiring (17, 17′) are typically implemented at M2 and M3 levels.
Recent advances in stress engineering, such as embedded SiGe structures and compressive stress liners, have increased on-current of PFETs much more than on-current of NFETs. This has resulted in degradation of writeability margins in existing SRAM cell designs because the NFET pass gate transistors have become relatively weak in comparison with pull-up PFETs during a write event.
A new SRAM design is a costly and time-consuming proposition since many rounds of thorough simulation and hardware verification may be necessary to optimize and verify all aspects of functionality of the new SRAM design. Therefore, methods of tuning SRAM performance by tuning process parameters are desired.
One approach to solve this problem may be to increase the widths of the pass gate NFETs and pull-down NFETs. This would result in an undesirable consequence of an increased cell size. Another approach may be to reduce the threshold voltage (Vt) of NFETs to address the writability concerns. However, this would result in an increased leakage current and standby power consumption. Yet another approach may be to weaken the pull-up PFETs by raising the threshold voltage of the pull-up PFETs. This would degrade cell stability and limit the operability of an SRAM array at a lower power supply voltage Vdd.
In view of the above, there exists a need for a structure having a reduced on-current for pull-up PFETs in an SRAM cell without affecting the other transistor parameters in a significant manner and methods of manufacturing the same.